In vivo determination of phenylalanine hydroxylase activity in cirrhotic patients with 2H-phenylalanine

The phenylalanine hydroxylase activities of 23 cirrhotic patients (8 compensated and 15 de-compensated) and 8 normal adults were measured with a deuterated phenylalanine loading test. After a single iv injection of 10 mg/kg labeled L-phenylalanine or 20 mg/kg labeled DL-Phe, the abundances of plasma Phe and Tyrosine were analysed with GC-MS. The activities of phenylalanine hydroxylase were estimated by the slope of Tyr abundance curve, the slope of Tyr/Phe abundance ratio curve, the area under Tyr abundance curve, and the area under Tye/Phe abundance ratio curve. The results revealed that the activity of phenylalanine hydroxylase was impaired in cirrhotic patients and the impairment was more severe in de-compensated than in compensated cirrhosis